Construction of new large power generating plants has not kept pace with growing electricity demand. For example, in the western and northeastern regions of the United States, electricity demand has outpaced supply from power generating plants. At the same time, customer demand for even more highly reliable power is growing across the nation. Even if a sufficient number of new generating plants were built, the country's aging transmission and distribution systems are unlikely to reliably deliver the increased power supply that is needed. Moreover, the cost of the upgrades required to enable today's power system to deliver the level of reliability being demanded is far in excess of what society has so far been willing to bear.
In this context, distributed energy resources (DER), small power generators typically located at customers sites where the energy they generate is used, have emerged as a promising option to meet customers current and future demands for increasingly more reliable electric power. DER can include electricity generators, energy storage, load control, and, for certain classes of systems, advanced power electronic interfaces between the generators and the distribution grid.
DER systems range in size and capacity from a few kilowatts up to 50 MW. They comprise a variety of technologies, both supply-side and demand-side, that can be located at or near the location where the energy is used.
DER devices can provide opportunities for greater local control of electricity delivery and consumption. They also enable more efficient utilization of waste heat in combined heat and power (CHP) applications boosting efficiency and lowering emissions. CHP systems can provide electricity, hot water, heat for industrial processes, space heating and cooling, refrigeration, and humidity control to improve indoor air quality and comfort.
Current trends in DER are toward small technologies. One DER technology is small gas-fired microturbines in the 25–100 kW range, which many expect can be mass produced at low cost. These devices—which are high-speed (50,000–100,000 rpm) turbines with air foil bearings—are designed to combine the reliability of on-board commercial aircraft generators with the low cost of automotive turbochargers. Microturbines rely on power electronics to interface with loads. Example products include: Allison Engine Company's 50-kW generator, Capstone's 30-kW and 60-kW systems, and Honeywell's 75-kW Turbogenerator.
Fuel cells are also well suited for distributed generation applications. They offer high efficiency and low emissions but are currently expensive. Phosphoric acid cells are commercially available in the 200 kW range, and solid-oxide and molten-carbonate cells have been demonstrated. A major development effort by automotive companies has focused on the possibility of using gasoline as a fuel for polymer electrolyte membrane (PEM) fuel cells. In 1997, Ballard Generation Systems formed a strategic alliance with Daimler-Benz and Ford to develop new vehicle engines using Ballard's PEM fuel cell. Fuel cell costs for these engines are expected to be $200 per kW. Fuel cell engine designs are attractive because they promise high efficiency without the significant polluting emissions associated with internal combustion engines. Many other major international companies are investing in fuel cells, including General Motors, Chrysler, Honda, Nissan, Volkswagen, Volvo, and Matsushita Electric.
Microturbines and fuel cells are a major improvement over conventional combustion engines in their emissions of ozone, particulate matter less 10 μm in diameter (PM-10), nitrogen oxide (NOx), and carbon monoxide (CO). The primary fuel for microsources is natural gas, which has few particulates and less carbon than most traditional fuels for combustion engines.
Microsources that effectively use waste heat can have CO emissions as low as those of combined-cycle generators. NOx emissions are mainly a consequence of combustion. Some traditional combustion fuels, notably coal, contain nitrogen that is oxidized during the combustion process. However, even fuels that contain no nitrogen emit NOx, which forms at high combustion temperatures from the nitrogen and oxygen in the air. Gas turbines, reciprocating engines, and reformers all involve high temperatures that result in NOx production. Microturbines and fuel cells have much lower NOx emissions because of their lower combustion temperatures.
Distributed resources include more than microturbines and fuel cells. Storage technologies such as batteries, ultracapacitors, and flywheels are important. Combining storage with microsources provides peak power and ride-through capabilities during system disturbances. Storage systems have become far more efficient than they were five years ago. Flywheel systems can deliver 700 kW for five seconds, and 28-cell ultracapacitors can provide up to 12.5 kW for a few seconds.
There is a significant potential for smaller DER (e.g., <100 kW/unit) with advanced power electronic interfaces which can be manufactured at a low cost, have low emissions, and be placed near the customers load to make effective use of waste heat, sometimes called microsources.
In general, there are two basic classes of microsources: DC sources, such as fuel cells, photovoltaic cells, and battery storage; and high-frequency AC sources, such as microturbines and wind turbines, which need to be rectified. In both of these classes of microsources, the DC voltage that is produced is converted to AC voltage or current at the required frequency, magnitude, and phase angle. In most cases, the conversion is performed by a voltage inverter that can rapidly control the magnitude and phase of its output voltage. Fundamental frequency in an inverter is created using an internal clock that does not change as the system is loaded. This arrangement is very different from that of synchronous generators for which the inertia from spinning mass determines and maintains system frequency. Inverter-based microsources, by contrast, are effectively inertia-less. As a result, basic system problems include controlling the power feeder from the grid, the microsource's response speed, the sharing and tracking of loads among the distributed resources, the reactive power flow, the power factor, and the system's steady-state and transient stability cannot be achieved using methods developed over time for synchronous generators.
Heretofore, adding a microsource to a distributed energy resource (DER) system has required modifying existing equipment in the system. Further, conventional control of microsources in such a system has been dependent on control of units already in the system. As such, power from a microsource in a DER system has not been independently controlled.
Thus, there is a need for microsource controls to insure that new microsources can be added to the system without modification of existing equipment, sources can connect to or isolate from the utility grid in a rapid and seamless fashion, reactive and active power can be independently controlled, and voltage sag and system imbalances can be corrected. Further, there is a need for control of inverters used to supply power based on information available locally at each inverter. Yet further, there is a need for a local controller at the microsource to enable “plug and play” operation of the microsource. In other words, there is a need to add microsources to a distributed energy resource system without changes to the control and protection of units that are already part of the system.